The invention relates to a process for growth of materials on a substrate, and more particularly, to a process for epitaxial growth where the lateral and vertical growth rates of the material is controlled by processing conditions.
Nitride-based semiconductors have been shown to be useful as the material basis for devices such as light-emitting diodes and semiconductor lasers and laser diodes. Unfortunately, the development of materials, such as Group III-V materials, has been hampered by problems in the processing technology of such materials. For example, the difficulties in forming high-quality, single-crystalline Group III-V materials, such as GaN, over large areas are well known.
Group III-V materials tend to form dislocations and cracks easily when grown on non-lattice-matched substrates. High densities (108-1010/cm2) of threading dislocations have typically occurred in epitaxially-grown GaN materials when grown on lattice-mismatched substrates. The lack of large single-crystal GaN wafers has generally forced epitaxial growth of GaN and other Group III-N to be performed on substrates such as sapphire (Al2O3), SiC, LiGaO2, Si(111), or other materials that can provide an orientational template for growth of the Group III-nitrides. The resulting epitaxial layers tend to be heavily dislocated. The existence of dislocations leads to remarkably deteriorated performance and results in a shortened service life. The dislocations increase the amount of nonradiative emission, reducing the light-emitting efficiency of light-emitting diodes and laser diodes made from these materials. These dislocations also shorten the functional device lifetime of laser diodes. Although threading dislocations have not prevented the development of high-brightness light-emitting diodes, the dislocations cause excessive reverse-bias leakage currents in p-n junction devices such as high-electron-mobility transistors, field-effect transistors and other electronic devices. Further, the dislocations lower the mobility of electrons and holes unfavorably, making it difficult to realize high-speed operation in electronic devices.
Substantial difficulties have been shown when attempting to obtain large-area crystals of any Group III-nitride that could provide suitable substrates for device fabrication. An attempt to utilize a single crystal of gallium nitride for a single crystal substrate has been reported in Jpn. J. Appl. Phys., Vol. 35 (1996), L77-L79. The size of the single crystal of gallium nitride synthesized was only up to about 2 mm square, which is too small to be of practical use in semiconductor operations.
One of the most important problems in the development of III-N devices is the lack of a lattice-matched substrate for growth of low-defect III-N layers. Sapphire (an aluminum oxide) and SiC have become the standard substrates of III-N growth, despite lattice mismatches of several percent. Such large mismatches lead to the formation of very high densities of threading dislocations (109/cm2). To reduce the threading dislocation density with such substrates, two processes that involve III-N regrowth have been developed. Epitaxial lateral overgrowth (ELO), sometimes referred to as lateral epitaxial overgrowth (LEO), and pendeo-epitaxy (PE), are two regrowth techniques that take advantage of the faster growth of GaN in the [11{overscore (2)}0] direction to produce lower dislocation densities (less than approximately 107/cm2). Nam et al. (O. Nam, M. Bremser, T. Zheleva, and R. Davis, Appl. Phys. Lett., 71(18), 1997, 2638-2640) describe the production of III-V semiconductor materials using ELO. This ELO method requires an initial growth of a III-N layer on a substrate, removal from the growth reactor, ex-situ processing, deposition of dielectric masks, and re-insertion into the growth reactor. Various etching and other processing steps are included. In Nam et al., an AlN buffer layer was deposited on a SiC substrate, followed by deposition of a GaN film, followed by SiO2 mask patterning, followed by lateral epitaxial overgrowth of the GaN layer. Lateral overgrowth was achieved at a temperature of 1000-1100xc2x0 C. The lateral growth rate relative to the vertical growth rate is controlled by the reactor temperature, pressure, reactant concentrations and flow rates and can be estimated by development of semiempirical expressions for a given reactor system (Kongetira, P., Neudeck, G. W., Takoudis, C. G., J. Vac. Sci Technol. B, 1997, 15(6), 1902-1907).
As with the ELO method, the pendeo-epitaxial method, described by Linthicum et al. (K. Linthicum, T. Gehrke, D. Thomson, K. Tracy, E. Carlson, T. Smith, T. Zheleva, C. Zorman, M. Mehregany, and R. Davis, MRS Internet J. Nitride Semicond. Res. 4S1, G4.9, 1999) and Zheleva et al. (T. Zheleva, S. Smith, D. Thomson, T. Gehrke, K. Linthicum, P. Rajagopal, E. Carlson, W. Ashmawi, and R. Davis, MRS Internet J. Nitride Semicond. Res. 4S1, G3.38, 1999), requires an initial growth of a III-N layer on a substrate, removal from the growth reactor, ex-situ processing, and re-insertion into the growth reactor. In the PE method, lateral growth of GaN films suspended from (11{overscore (2)}0) side walls of (0001) oriented GaN columns into and over adjacent etched wells has been achieved via metal-organic vapor phase epitaxy (MOVPE) without the use of, or contact with, a supporting mask or substrate. Although not requiring the use of a SiO2 mask as with ELO, the pendeo-epitaxial method requires the growth of the seed GaN layer on a substrate, removal from the growth reactor for selective etching to form substrate columns and trenches in the substrate and growth of the epitaxial GaN layer. This method also has growth off a high-dislocation density substrate.